The present invention relates to a new and improved construction of an energy generation installation, i.e. a power generating plant, of the type equipped with a gas-cooled nuclear reactor having a closed gas circulation system which extends through at least one compressor, the reactor, through a first and second gas turbine and back to the compressor, and wherein as concerns the gas turbines at least a respective one is in driving connection with the compressor and the other is in driving connection with an electric generator for the delivery of electrical energy, and there is further provided a recuperative heat exchanger having a primary path located between the outlet of the second gas turbine and the inlet of the compressor.
With prior art power generating plants of the abovementioned type, the nuclear reactor is located in the gas-flow path between the compressor and the first gas turbine, and for improving the efficiency of the reactor the secondary path of the recuperative heat exchanger can be forwardly arranged for the purpose of pre-heating the gas.
The pressure conditions which exist in the gas circulation system of such plants can lead to considerable temporary pressure gradients during transient effects, and which particularly occur during regulating functions and during starting-up and shutting down of the plant. A particularly rapid drop in pressure occurs at the nuclear reactor during load decrease of the generator, that is to say, during sudden shutting-off of the electrical energy consumer. In order to be able to reduce to null the power delivered by the gas turbines rapidly enough, there must be opened a valve in a return flow path which is connected parallel to the compressor. As a result, the pressure conditions which prevail at the gas circulation system drop to the considerably lower no-load value. In the case of helium turbines which are employed for nuclear power plants, this entails a reduction in pressure at the turbine inlet and therefore in the reactor in the order of magnitude of 10 bar within a fraction of a second. Such pressure gradients can lead to extensive damage to the internal insulation of the reactor as well as to the structure of the reactor core.
It is known to the art that for plants or installations of the previously mentioned type it is possible to suppress the reduction in pressure in the reactor as a result of a load decrease at the generator if for the drive of the compressor on the one hand and for the drive of the generator on the other hand there are employed two gas turbines with separate shafts. However, a precondition for this arrangement is the reliable functioning of the requisite automatic regulating elements. Yet, also with this construction of the power plant there can not be avoided an undesired rapid pressure reduction at the reactor during quick shutdown of the turbine or during bucket damage or other damage to the turbo-machines.
Gas-cooled fast breeder reactors possess a small thermal storage capability because they do not contain any neutron moderator. Furthermore, they operate with very high power density and owing to their hard neutron spectrum possess a very short longevity of the prompt neutrons. These properties require that during rapid shutoff of such reactor it is necessary to immediately considerably reduce the cooling gas stream, since otherwise by under cooling the reactor there can occur impermissibly great temperature gradients. A reduction of the cooling gas stream can be realized by reducing the rotational speed of the turbo assembly, which however will result in a rapid pressure reduction at the reactor. An even more pronounced pressure reduction has been obtained during attempts to reduce the coolant stream by reducing the total gas quantity in the circulation system. If a bypass conduit which is connected parallel to the nuclear reactor is opened, in order to reduce the flow of the coolant gas stream through the reactor, then such produces a reduction in the rotational speed of the turbo assembly and therefore also a drop in pressure in the reactor.
Large pressure fluctuations are not only undesired in gas-cooled fast breeder reactors owing to the already mentioned danger of damaging the insulation and the reactor core structure, but also because of their positive influence upon the reactivity of the nuclear reactor, that is to say, owing to the therewith attendant reduction in the action of the moderator, for the compensation of which there is necessary an immediate adjustment at the reactor regulation system. On the other hand, the Doppler effect of the reactor upon the absence of the pressure changes of the coolant gas in the reactor should contribute completely to increasing the reactor stability.
From what has been discussed above, it should be apparent that in energy generating plants equipped with a gas-cooled nuclear reactor with a closed gas circulation system, it is of decisive importance that the coolant gas pressure in the nuclear reactor remain as constant as possible when, at other locations of the gas circulation system, there occur pressure equalization effects as such is especially the case during sudden shutting-off of the electrical energy consumer from the generator or during disturbances at the turbo assembly.